Implement Phantom typing for Tiny FastCache layer

Refactor FastCache and TLS cache APIs to use Phantom types (hak_base_ptr_t)
for compile-time type safety, preventing BASE/USER pointer confusion.

Changes:
1. core/hakmem_tiny_fastcache.inc.h:
   - fastcache_pop() returns hak_base_ptr_t instead of void*
   - fastcache_push() accepts hak_base_ptr_t instead of void*

2. core/hakmem_tiny.c:
   - Updated forward declarations to match new signatures

3. core/tiny_alloc_fast.inc.h, core/hakmem_tiny_alloc.inc:
   - Alloc paths now use hak_base_ptr_t for cache operations
   - BASE->USER conversion via HAK_RET_ALLOC macro

4. core/hakmem_tiny_refill.inc.h, core/refill/ss_refill_fc.h:
   - Refill paths properly handle BASE pointer types
   - Fixed: Removed unnecessary HAK_BASE_FROM_RAW() in ss_refill_fc.h line 176

5. core/hakmem_tiny_free.inc, core/tiny_free_magazine.inc.h:
   - Free paths convert USER->BASE before cache push
   - USER->BASE conversion via HAK_USER_TO_BASE or ptr_user_to_base()

6. core/hakmem_tiny_legacy_slow_box.inc:
   - Legacy path properly wraps pointers for cache API

Benefits:
- Type safety at compile time (in debug builds)
- Zero runtime overhead (debug builds only, release builds use typedef=void*)
- All BASE->USER conversions verified via Task analysis
- Prevents pointer type confusion bugs

Testing:
- Build: SUCCESS (all 9 files)
- Smoke test: PASS (sh8bench runs to completion)
- Conversion path verification: 3/3 paths correct

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

core/tiny_alloc_fast.inc.h

@@ -389,12 +389,13 @@ static inline void* tiny_alloc_fast_pop(int class_idx) {
     // Phase 1: Try array stack (FastCache) first for hottest tiny classes (C0–C3)
     // Phase 7-Step4: Use config macro for dead code elimination in PGO mode
     if (__builtin_expect(TINY_FRONT_FASTCACHE_ENABLED && class_idx <= 3, 1)) {
-        void* fc = fastcache_pop(class_idx);
-        if (__builtin_expect(fc != NULL, 1)) {
+        hak_base_ptr_t fc = fastcache_pop(class_idx);
+        if (__builtin_expect(!hak_base_is_null(fc), 1)) {
+            void* fc_raw = HAK_BASE_TO_RAW(fc);
             // Frontend FastCache hit (already tracked by g_front_fc_hit)
             extern unsigned long long g_front_fc_hit[];
             g_front_fc_hit[class_idx]++;
-            return fc;
+            return fc_raw;
         } else {
             // Frontend FastCache miss (already tracked by g_front_fc_miss)
             extern unsigned long long g_front_fc_miss[];